LR 308 timing or HP bolt

[Danimal]

Aero M5E1 set with ballistic advantage 24" 6.5 creedmoor, rifle length barrel. DPMS bolt and standard 308 rifle spring and buffer in a magpul PRS. Hit a milk jug 3 out of 10 at 1000 yards, so its a shooter.

The problem is that it is not cycling reliably and blowing primers on everything. Factory ammo is blowing primers about 50% of the time so I am leaning toward timing problems.

I know 6.5 CM have HP bolts available, and I will probably get one but I want to fix the cycling problem first. I installed a JP adjustable gas block, it is wide open and will eject, but not load next round. I verified that it sealed well to the barrel, and I see no gas leaks anywhere. From what I understand, 6.5 is supposed to be overgassed on the 308 platform.

So now I am wondering if the bolt unmatched before the bullet has exited causing the blown primer, and a lot of force and energy is spent grinding the bolt against the lugs and then there is not enough energy to stroke the bolt?

Strangely I had some reloads that were really slow while working up and they cycled the gun perfectly. As soon as I reach manufacturer speeds around 2700 fps the gun can't get the next round in, and adjusting the gas block did not seem to help.

Any ideas are welcome, or things to check. Will post pics if you have anything specific to check.

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[Hoser]

Get a heavier buffer and a new firing pin. The tip is most likely junk now.

If you still pop primers, the chambers leade might be too short.

Fritz at Mile High can help you there.

[Danimal]

Get a heavier buffer and a new firing pin. The tip is most likely junk now.

If you still pop primers, the chambers leade might be too short.

Fritz at Mile High can help you there.Chamber passed go no go, brass trimmed to .005 over SAAMI min (1.905") cases chamber easily and bolt can close and lock. Bullets seated to .02 off of lands for a OAL of 2.825. Factory cases that measure 1.915 can chamber easily, so I am sure the leade is correct.

Does that offset seem like enough? I have always been told that 0.01 is the minimum offset from the rifling, but this is the first rifle that I have owned that I can get less than 0.04 and fit the bullet in a magazine.

The firing pin tip does look like crap, but I have been holding off because I might buy a JP HP bolt with the smaller firing pin.

Can I get your opinion on the JP low mass BCG? It would do the exact opposite of what you suggested, but I just wonder if it would allow me to turn down the gas block and get reliable function.

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[Danimal]

Quick followup on this for others in the future. 6.5 is way over gassed for the LR 308 AR10 platform. It will show all of the signs of over pressure with the exception of loose primer pockets. If you are getting loose primer pockets, that's a real over pressure.

Timing is a very small window to get it right, it is easy to shoot right past it if you are not careful. It will continue to operate "okay" but not reliably and soon you will start blowing primers. The blown primers are caused by the bolt opening before the bullet leaves the barrel and the slight relief in pressure causes the primer center to blow out. As this happens the operation degrades and it stops reliably cycling, which might make you think that you need to open the gas port further, but that will just make things much worse.

I finally got everything working by taking much smaller steps on my JP gas block. Putting only one round in the mag, I started on turn open and opened one quarter turn until I had bolt hold open on last round. I then shot 5 rounds in a group to check function. In my case I needed to open about 1/4 more open to get it operating correctly. The instructions for the block state 1/2 turn increments, and after bolt hold open, open one additional 1/2 turn. This caused me to blow right past proper function and into over gas.